U.S. patent number 4,699,849 [Application Number 06/756,008] was granted by the patent office on 1987-10-13 for metal matrix composites and method of manufacture.
This patent grant is currently assigned to The Boeing Company. Invention is credited to K. Bhagwan Das.
United States Patent |
4,699,849 |
Das |
October 13, 1987 |
Metal matrix composites and method of manufacture
Abstract
A metal matrix composite is produced by plastically deforming a
metal powder, either before or after blending the powder with
ceramic fibers, and compacting the mixture at elevated temperatures
to achieve substantially full density. Imparting strain energy to
the metal allows reduction of the compaction temperature to
eliminate reaction between the fibers and the metal or degradation
of the fibers. Silicon nitride fibers are thermodynamically
superior for use in aluminum or titanium metal matrix composites,
since silicon nitride fibers are more stable at the temperatures
required for full compaction. Secondary phase reactions are
avoided.
Inventors: |
Das; K. Bhagwan (Seattle,
WA) |
Assignee: |
The Boeing Company (Seattle,
WA)
|
Family
ID: |
25041636 |
Appl.
No.: |
06/756,008 |
Filed: |
July 17, 1985 |
Current U.S.
Class: |
428/698; 428/379;
428/386; 428/388; 428/389; 428/450; 428/469; 428/902 |
Current CPC
Class: |
C22C
47/14 (20130101); C22C 49/04 (20130101); C22C
49/11 (20130101); Y10T 428/2953 (20150115); Y10T
428/2958 (20150115); Y10T 428/294 (20150115); Y10T
428/2956 (20150115); Y10S 428/902 (20130101) |
Current International
Class: |
C22C
49/00 (20060101); C22C 49/04 (20060101); C22C
47/00 (20060101); C22C 47/14 (20060101); C22C
49/11 (20060101); B22F 001/00 (); B22F
009/00 () |
Field of
Search: |
;428/698,699,379,386,388,389,450,469,902 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Swisher; Nancy A.
Attorney, Agent or Firm: Hammar; John C.
Claims
What is claimed is:
1. A metal matrix composite comprising:
a matrix metal selected from the group of alloys consisting of
titanium and aluminum alloys, the metal having been compacted to
near full theoretical density at elevated temperatures and
pressure; and
silicon nitride fibers uniformly dispersed throughout the matrix
metal, the composite being further characterized by the absence of
substantially any evidence of interfacial reaction between the
metal and the fibers.
2. The composite of claim 1 wherein the fibers comprise about 10 to
20% by volume of the composite.
3. The composite of claim 2 wherein the metal is a titanium
alloy.
4. The composite of claim 2 wherein the metal is an aluminum
alloy.
5. The composite of claim 2 wherein the fibers are uncoated.
6. The composite of claim 1, wherein the fibers are whiskers.
7. A metal matrix composite comprising an aluminum alloy or
titanium alloy and ceramic fiber whiskers formed by compacting a
substantially uniform dispersion of matrix metal and fibers to
substantially full theoretical density at an elevated temperature
and pressure, the composite being characterized by having
essentially no evidence of secondary phase reaction at the
interface between the metal and fibers.
8. The composite of claim 7 wherein the whiskers are silicon
nitride.
9. The composite of claim 8 wherein the alloy is an aluminum
alloy.
10. The composite of claim 8 wherein the alloy is a titanium alloy,
and wherein the alloy is plastically deformed prior to compacting
to reduce the compacting processing temperature.
11. The composite of claim 7 wherein the fibers comprise about
10-20 volume % of the composite.
12. The composite of claim 10 wherein the fibers are uncoated.
13. The composite of claim 7 wherein the fibers are uncoated.
14. A metal matrix composite comprising a substantially full
density compact including Ti-10-2-3 alloy and about 10-20 volume %
silicon nitride fibers substantially uniformly dispersed throughout
the titanium alloy, the composite being further characterized by
having essentially no evidence of secondary phase reaction at the
fiber/metal interface.
15. The composite of claim 14 wherein the fibers are discontinuous
whiskers having an aspect ratio of about 20-200.
16. A metal matrix composite consisting essentially of a titanium
alloy matrix metal and Si.sub.3 N.sub.4 whiskers, the titanium
alloy matrix metal being plastically deformed prior to compaction
with the whiskers, the composite being characterized by essentially
no secondary phase reaction at the interface between the metal and
whiskers and being of substantially full theoretical density.
Description
TECHNICAL FIELD
The present invention relates to metal matrix composites, and
particularly to composites reinforced with silicon nitride
fibers.
BACKGROUND ART
Ceramic reinforcement in metal matrices improves the properties or
functional characteristics of various metals and alloys. Chopped or
continuous fibers, whiskers, or particulates can be used as
reinforcement matrix metals to enhance the specific strength (i.e.
strength/density), specific modulus (i.e. modulus/density), and the
temperature service capabilities of the composites. Improvement in
the specific strength is achievable both by reducing the density
and by increasing the absolute strength and modulus through the
introducton of the ceramic reinforcement. The result is typically a
composite providing a significant weight reduction for components
having critical strenth or stiffness requirements For example, a
metal matrix composite containing 80 volume % aluminum and 20
volume % silicon carbide has a stiffness comparable to steel, but
is considerably lighter. Furthermore, the composite has improved
corrosion resistance over steel.
Metal matrix composite research has focused on the development of
aluminum based composites using boron, borsic, graphite, or silicon
carbide reinforcement in particulate, continuous fiber, or discrete
fiber forms. Continuous fibers offer the potential of highly
anisotropic properties in the composite by aligning the fibers in
primarily one direction. Unfortunately, the off-axis properties of
these composites have proven to be quite low. Discontinuous or
discrete fibers, however, offer greater potential for tailoring the
properties of the composite. For example, by cross-rolling a SiC-Al
composite the composite can possess nearly isotropic properties
while the same composite may be highly anisotropic if prepared by a
multiple extrusion process or if worked with only unidirectional
rolling. The degree of stiffness anisotropy can be controlled over
a wide range.
Forming composites with continuous or very long fibers often
requires highly specialized fabrication techniques to avoid (1),
fiber breakage, (2) fiber bunching, (3) nonuniform fiber/matrix
interfacial bonding, or (4) void concentrations. Whiskers or
particulates are more readily used, particularly in powder
metallurgy, casting, hot extrusion, rolling, and forging.
Machining, drilling, grinding, joining, and other operations are
also more readily accomplished with composites having discrete or
discontinuous fibers, since the properties of the composite are not
as severely linked to the continuity of the fiber.
When using powder metallurgy to fabricate composites, the metal
matrix powder is blended with the fiber and is cold pressed to form
a green compact structure. The green structure is then vacuum
compacted or isotactically pressed at elevated temperatures and
pressures to cure the green structure and to achieve full density
in the composite. Full density is necessary to ensure the integrity
of the article and to attain the necessary mechanical properties.
Unfortunately, the high temperatures required for vacuum compaction
to full density can lead to adverse reaction between the fibers and
matrix metal, especially for SiC fibers in reactive metals like
aluminum and titanium. Such reaction affects the integrity of the
composites and their mechanical properties. Secondary phases, such
as carbides, borides, silicides or nitrides, can be formed in these
reactive composites, and are predictable based upon thermodynamic
considerations. Reducing the deleterious reaction between the
fibers and matrix is a necessary improvement to metal matrix
composite technology.
U.S. Pat. Nos. 4,073,648 (Volin et al.) and 3,976,482 (Larson)
disclose inducing strain energy in prealloyed metal powder to
improve thermoplasticity of the powder used in specialty
superalloys, particularly in powder metallurgy (P/M).
Methods for forming metal matrix composites are illustrated in U.S.
Pat. Nos. 3,546,769; 4,060,412; and 4,259,112.
SUMMARY OF THE INVENTION
Loss of mechanical properties in the metal matrix composites of
reactive metals is achieved with the selection of silicon nitride
fibers that are thermodynamically superior to other reinforcements.
Powder metallurgy and vacuum hot compaction techniques can be used
without stimulating adverse reactions between the matrix metal and
fibers.
By imparting strain energy to the matrix metal, the processing
temperature can be reduced, thereby reducing further the risk of
adverse fiber/matrix reactions or fiber degradation. Fully dense
composites can readily be formed with conventional processing
techniques, but at lower temperatures.
The preferred process of the present invention comprises the steps
of plastically deforming the matrix metal to impart significant
strain energy to the metal, mixing the strain energized metal with
ceramic fibers (preferably having an aspect ratio (1/d) of 20-200),
and compacting the mixture at elevated temperatures to form a metal
matrix composite of substantially full theoretical density. The
strain energy stored in the metal allows the compaction to occur at
lower temperatures so that adverse reactions do not occur between
the fibers and the matrix metal. The required microstructure of the
matrix metal is achieved, however, as well as substantially full
density. Preferably, the matrix metal is a titanium or aluminum
alloy, and the fibers are silicon nitride. Compacting for titanium
can, then, occur at a temperature of about 500.degree. to
700.degree. C. and at a pressure of about 50 KSI. For aluminum
metal matrix composites, compacting can occur between 500.degree.
to 600.degree. C. at a pressure of from 20 to 40 KSI.
The matrix metal and fibers can be mixed prior to imparting the
strain energy to the metal. Premixing can result in some breakage
of the fibers during the milling, and in reduced mechanical
properties. Preferably, the metal is plastically deformed by
milling prior to addition of the fibers. Ball milling reduces the
likelihood of agglomeration which can occur and which should be
avoided.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention includes a process for manufacturing a metal
matrix composite without heating the materials to a point that the
fibers degrade or react with the metal. Where the metal of the
matrix is an alloy, the metal powder should be pre-alloyed.
Titanium and aluminum alloys, such as Ti-10V-2Fe-3Al or CT90 or
7090 aluminum alloy, having 8% zinc, 2.5% magnesium, 1% copper,
1.4% cobalt, and the balance essentially aluminum, may be used in
this process. These titanium and aluminum alloys are particulary
reactive, so the problems of adverse interfacial reaction between
the metals and fibers is particularly acute. The thermodynamic and
strain energy concepts of this invention are particularly important
for making metal matrix composites from these types of alloys. By
plastically deforming the metal to impart significant strain energy
to the particles, the temperature of the compaction to achieve full
density can be reduced so that the risk of degradation of the
fibers or reaction between the fibers and metal is eliminated.
The ceramic fibers usually are silicon nitride whiskers made
according to the process of U.S. Pat. No. 4,388,255 to Simpson or
of Verzemnieks in his copending patent application, U.S. Ser. No.
No. 536,962. An aspect ratio (1/d) of 20-200 is preferred.
Silicon nitride fibers exhibit a standard free energy of formation
far more negative than aluminum nitride, titanium nitride, or
titanium silicide up to at least about 1400.degree. C. Thus, the
fibers are thermodynamically more stable than the reactive metal
nitrides so that secondary phases are less likely to form during
high temperature processing of aluminum or titanium based metal
matrix composites. In contrast, silicon carbide has a more positive
free energy than these secondary phases, indicating that aluminum
carbide and titanium carbide are likely to form at the elevated
processing temperatures.
The matrix metal can be plastically deformed to impart the desired
strain energy in a number of ways. Spherical, prealloyed metal
particles can be passed through opposed rolls to impart the
requisite strain energy. For titanium particles, reducing the
diameter by approximately 60 to 80% has proven successful.
Deformation to achieve the strain energy can occur even after the
particles are mixed with the fibers to form a metal/fiber mixture.
The fibers and metal are blended to form a substantially uniform
dispersion so that the physical properties of the resulting article
will be uniform. Agglomeration of the fibers during the blending
should be avoided. Vibrating the mixture has proven as one means to
achieve the desired dispersion.
Plastic deformation (or strain energizing) aids microstructural
refinement of the composite during compaction through
recrystallization caused by the cold working. The deformation also
reduces the effective compaction temperature necessary to achieve
full density since diffusion rates of the metal and its flowability
are enhanced.
The blended mixture is compacted at elevated temperatures to form a
metal matrix composite of substantially full density. Compaction
can occur in several steps, and usually entails cold pressing to
form a green structure. For titanium alloys, the compacting step
can be carried out at a temperature of about 500.degree. to
700.degree. C. and a pressure of about 50 KSI (50,000 lbs. per
square inch). For aluminum alloys, it is preferred to compact the
material at a temperature of about 500.degree. to 600.degree. C.
and a pressure of about 20 to 40 KSI. The maximum compaction
temperature depends on the particular alloy and should be below the
solidus temperature of the alloy. The compaction pressure depends
on the alloy and the morphology of the fibers.
Plastic deformation may not be necessary for aluminum alloy metal
matrix composties including silicon nitride fibers, since the
alloys have relatively low melting points and are softer than
titanium alloys. Even without imparting strain energy to these
matrix metals, the processing temperatures may remain low enough
that the alloy and silicon nitrides fibers will not react and the
fibers will not degrade.
Hot isostatic pressing in a gas pressurized vessel to reach full
theoretical density is preferred. Where the article is compacted to
an intermediate density prior to compaction to full theoretical
density, the initial compaction may either be by hot isostatic
pressing, cold pressing at room temperature, or by mechanical
compaction where the configuration of the article is amenable to
shaping by means of mechanical tooling. Cold pressing is
preferred.
The strain energy imparted to the metal allows compaction to full
density without detrimental reaction or degradation of the fibers.
Heating is required to achieve the desired microstructure of the
composite. By imparting strain energy, the temperature can be
reduced while the desired properties can be achieved in the
composite.
Secondary phases such as aluminum carbide, titanium carbide,
titanium silicide, aluminum nitride, or titanium nitride, are
brittle phases, and are undesirable in the composites. From a
thermodynamic point of view, silicon nitride is far superior to
silicon carbide as a fiber candidate for metal matrix composites.
Of course, the kinetics of secondary phase reactions must also be
considered when selecting a suitable fiber as well as the
processing technique. Silicon carbide may be adequate if the
processing conditions are such that there is inadequate time for
secondary phase adverse reactions to occur.
Silicon carbide-titanium metal matrix composites are subject to
stress cracking at high temperature. Silicon nitride-titanium
composites of the present invention avoid these problems, exhibit
superior strength to density ratios (specific strength), and can be
used in applications requiring exposure of high temperatures up to
and above 2200.degree. F. These composites of silicon
nitride-titanium present substantial weight savings over steels
while providing comparable strength and stiffness.
TABLE I ______________________________________ Mechanical
Properties of High-Strength CT90 (X7090) Aluminum Alloy Matrix
Composites Reinforced With Si.sub.3 N.sub.4 and SiC (Fibers and
Particulates) Total Strain Ultimate to Reinforcement Modulus
Strength Failure Material (10.sup.6 psi) (ksi) (%)
______________________________________ 20 Vol. % Si.sub.3 N.sub.4
15.0 28.0 0.27 Fibers 20 Vol. % Si.sub.3 N.sub.4 16.3 30.1 0.22
Particulates 20 Vol. % SiC (F-9) 16.4 42.7 0.44 Fibers 20 Vol. %
SiC 16.8 77.3 0.64 Particulates 20 Vol. % SiC 16.8 40.1 0.52 Fibers
(Great Lakes) ______________________________________
Table I shows the mechanical properties of a silion
nitride-aluminum composite made in accordance with the invention
compared to an aluminum composite having silicon carbide
fibers.
With the method of the present invention comparable mechanical
properties were achieved. The composites had about a 50% increase
in modulus over the unreinforced CT90 or 7090 aluminum alloy. An
examination of the microstructure of the silicon nitride composite
showed no evidence of interfacial reaction between the fiber and
the matrix metal.
In Table I, the aluminum alloy composition (in weight %) was 8%
zinc, 2.5% magnesium, 1% copper, 1.4% cobalt, and balance aluminum.
Test panels of approximately 8.times.5.times.0.05 inches were
produced by hot pressing in a die cavity at a temperature of
565.degree. C. The volume fraction of the reinforcing material was
in all cases approximately 20%.
A titanium based alloy of the composition, in weight %, 10%
vanadium, 2% iron, 3% aluminum, and balance titanium had the
particle size distribution of Table II.
TABLE II ______________________________________ Ti-10-2-3 Strain
Energized Power (SEP) Sieve Analysis WEIGHT % U.S. STANDARD MESH
SIZE Can #1 Can #2 ______________________________________ +20 1.7
1.3 +40 17.0 10.0 +100 65.1 71.9 +200 13.6 15.6 +400 2.5 1.3
______________________________________
This alloy was roll milled prior to blending with fibers to impart
strain energy to the particles by a 60-80% reduction in particle
diameter. Silicon nitride and silicon carbide fibers,
TABLE III ______________________________________ FIBER-PARTICULATE
MORPHOLOGY Fiber Length Fiber Diameter Particulate (microns)
(microns) (microns) Mean Max Mean Max Mean Max
______________________________________ Si.sub.3 N.sub.4 11.1 76
0.37 1.35 15.5 77.5 SiC 16.5 105 1.35 -- 5.7 20
______________________________________
characterized as set forth in Table III, were uniformly dispersed
in the titanium by vibrating the mixture so that the fibers
comprised 10 volume % of the mixture.
Mixtures were loaded into a one-inch diameter die and were cold
pressed to achieve a green strength suitable for handling the
billet. Each billet was then vacuum hot pressed to obtain a
one-inch diameter billet. The compaction conditions of temperature
and pressure and resulting composite densities were as set forth in
Table IV.
TABLE IV
__________________________________________________________________________
VACUUM HOT PRESSED Ti-- 10V--2Fe--3Al + 10 V/O REINFORCEMENT
CONSOLIDATION CONSOLIDATION BULK FIBER TEMPERATURE PRESSURE DENSITY
MATERIAL (.degree. C.) (KSI) (% THEORECTICAL)
__________________________________________________________________________
SiC 676 TOOL FAILURE 88.0 Si.sub.3 N.sub.4 619 38.4 99.3 Si.sub.3
N.sub.4 580 53.2 97.9 SiC 580 51.2 78-Irregular Shape Si.sub.3
N.sub.4 540 51.2 85.5 SiC 540 51.2 85.7
__________________________________________________________________________
Metallographic examination of the billets showed a uniform
dispersion of fibers in the matrix. There was no evidence of
chemical reaction between the matrix and the silicon nitride
fibers.
These examples show that:
(1) Whisker reinforced Ti-10-2-3 have been successfully
consolidated to near theoretical density.
(2) The mixing/blending has been successful in achieving a fairly
uniform dispersion of reinforcing fibers in the titanium powder
matrix.
(3) The degree of chemical reaction between the reinforcement and
the matrix has been minimized through the use of strain energized
titanium powders and the concomitant decrease in the processing
temperature.
While preferred embodiments have been described, those skilled in
the art will readily recognize variations, modifications, or
alterations which might be made to the embodiments without
departing from the inventive concept. Therefore, the invention
should be interpreted broadly. The examples are meant to illustrate
the invention and not to limit it. The claims should be interpreted
broadly to cover the invention and should only be limited as is
necessary in view of the pertinent prior art.
* * * * *